Metal-insulator-silicon (MIS) transistors, including metal-oxide-silicon (MOS) transistors, are comprised of doped source and drain regions formed in the surface of a semiconductor substrate, a channel region between the source and drain, and a gate electrode situated over the channel region. The gate electrode is physically and electrically separated from the channel by a thin gate dielectric (oxide) layer, typically silicon dioxide. The gate electrode typically comprises a doped polysilicon material. Diffusion of dopants such as boron from the doped polysilicon gate through the gate oxide layer into the underlying silicon substrate poses serious problems in processing and the functioning of the device.
To inhibit boron diffusion, nitrogen has been incorporated into the gate oxide layer. One conventional method of incorporating nitrogen into the oxide layer is by anneal of the oxide layer in nitric oxide (NO), nitrous oxide (N2O), ammonia (NH3) or other nitrogen-containing species. However, thermal nitridation of the gate oxide layer results in nitrogen incorporation at the silicon/oxide interface, which increases the ability of the gate oxide layer to suppress boron penetration but can result in transconductance loss.
Another method of forming a nitrided gate oxide layer is by remote plasma nitridation by exposing the surface of the oxide layer to a plasma generated species of nitrogen. This results in the polysilicon/oxide interface being nitridized as opposed to the gate oxide/silicon interface, thus avoiding transconductance loss. However, data indicates that the plasma nitridation may not be scaleable below 25 angstroms for integrated circuit (IC) devices with high processing thermal budgets such as DRAMS or flash devices due to the loss of integrity of the gate oxide as well as the loss of transconductance due to the proximity of nitrogen to the gate oxide-silicon interface.
Another conventional method to incorporate nitrogen into the gate oxide layer is to form a composite gate dielectric layer comprising a silicon nitride layer and an oxide layer. An issue with forming such a composite gate oxide is that the interface between the silicon nitride and oxide layers typically requires rigorous post-treatment processing to eliminate potential sources of charge trapping. In addition, composite gate dielectrics that comprise nitride and thermal oxides have limitations due to the total effective oxide thickness that can be achieved due to poor nucleation of nitride on oxide. This requires the formation of a relatively thick nitride layer resulting in an overall effective oxide thickness that is higher than that which is considered as usable.
Thus, a need exists for a nitride barrier layer that avoids such problems.